1. Field of the Invention
The present invention relates to a device for tuning a frequency so that a high frequency wave (HF) is resonated in a resonant cavity.
2. Description of the Related Art
There is known a device that generates the high frequency wave resonated in a cavity to accelerate electrons by the high frequency wave. The accelerated electrons are irradiated to a metal target, for example, to be used for generating X-rays. The generated X-rays are irradiated to a diseased part of a human body in a radiotherapy apparatus.
In order for the high frequency wave to resonate in a cavity, it is necessary for the frequency of the high frequency wave to match with that of the resonant frequency in the cavity with a high accuracy. The resonant frequency changes depending on the environmental conditions, fluctuations in the temperatures caused by introducing the high frequency to the cavity, and the like. Therefore, it is necessary to adjust the frequency assuming that there is shift in the resonant frequency.
In a published textbook (C. J. Karzmark, Craig S. Nunan and Eiji Tanabe, 1993, Medical Electron Accelerators, McGraw-Hill, New York), a technique for tuning the frequency of a microwave to be resonated in an acceleration cavity is described. This conventional technique is capable of performing a control when a phase difference between a phase of traveling wave towards the acceleration cavity and a phase of reflection wave that is reflected from the acceleration cavity falls within a narrow range of −90 degree to 90 degree, however, it is not capable of performing the control when the phase difference is beyond that range. In addition, the range of the high frequency value falling within the range of the phase difference, −90 degree to 90 degree, is extremely narrow especially for the acceleration cavity with a high Q-value. Thus, it is difficult to achieve the controllable phase difference within the range of −90 degree to 90 degree.